Rapid development of contrast agents in the recent years has generated a number of different compositions and formulations, which are useful in contrast-enhanced imaging of organs and tissue of human or animal body as well as in therapeutic treatments thereof.
A class of contrast agents, particularly useful for ultrasound contrast imaging, includes suspensions of gas bubbles of nano- and/or micro-metric size dispersed in an aqueous medium. The gas is typically entrapped or encapsulated in a stabilizing film layer comprising, for instance, emulsifiers, oils, thickeners or sugars. These stabilized gas bubbles (dispersed in a suitable physiological solution) are generally referred to in the art with various terminologies, depending typically from the stabilizing material employed for their preparation; these terms include, for instance, “microspheres”, “microbubbles”, “microcapsules” or “microballoons”, globally referred to here as “gas-filled microvesicles” (or “microvesicles”). Of particular interest are aqueous suspensions of gas-filled microvesicles where the bubbles of gas are bounded at the gas/liquid interface by a very thin envelope (film) involving a stabilizing amphiphilic material (typically a phospholipid) disposed at the gas to liquid interface. These suspensions are advantageously prepared by contacting powdered amphiphilic materials, e.g. freeze-dried preformed liposomes or freeze-dried or spray-dried lipid solutions, with air or other gas and then with an aqueous carrier, while agitating to generate a suspension of gas-filled microvesicles which can then be administered, preferably shortly after its preparation. Examples of aqueous suspension of gas-filled microvesicles and preparation thereof are disclosed, for instance, in U.S. Pat. Nos. 5,271,928, 5,445,813, 5,413,774, 5,556,610, 5,597,549, 5,827,504, WO 97/29783 and WO2004/069284, which are herein incorporated by reference in their entirety. The stabilizing layer may comprise, in addition to the above cited phospholipids, also other amphiphilic materials, such as fatty acids. For instance, Sonovue® (an ultrasound contrast agent commercialized by Bracco Suisse S.A.) comprises a mixture of phospholipids and fatty acid as the film-forming stabilizing layer.
More recently, attention has been given to so-called “molecular imaging”, where suitable target specific components are used in the formulation of the contrast agents, for allowing selective contrast-enhanced imaging of organs or tissues. In addition, therapeutic use of contrast agent formulations, optionally in combination with molecular imaging, has also been described.
The formulations of gas-filled microvesicles may be suitably modified, either for improving the diagnostic effect (e.g. through molecular imaging) and/or for therapeutic purposes, such as drug delivery and/or ultrasound mediated thrombolysis. For instance, microvesicles may be associated (e.g. by inclusion in their boundary envelope) with therapeutic agents and/or with specific components which are capable to link to a determined target within a patient's body (known as “targeting ligands”). Examples of targeting ligands include, for instance, peptides, proteins, antibodies, aptamers or carbohydrates capable of binding to specific receptors expressed by organs or tissues during pathogenic processes such as, for instance, angiogenesis, inflammation or thrombus formation.
The Applicant has now found that it is possible to improve certain characteristics of phospholipid-based gas-filled microvesicles by including in the formulation of the stabilizing layer a mixture of saturated and unsaturated fatty acids. In particular, it has been observed that the presence of said mixture of saturated/unsaturated fatty acids surprisingly increases the stability of the gas-filled microvesicle with respect to corresponding microvesicles containing only one of the respective saturated or unsaturated fatty acid in the formulation forming the stabilizing layer.